It is proposed to apply the most advanced and promising quantitative procedures to the study of platelet and fibrin thromboembolism, as well as the hemostatic reactions, in living blood vessels of the microvascular system, and to develop effective means of detecting and controlling these reactions. Investigations will focus on the biophysical and biochemical changes that occur during thromboembolism and hemostasis. Three major approaches will be utilized: in vivo experimentation with emphasis placed on direct microscopic observation and quantitative production and analysis of events; development and application of new instrumentation pertinent to the in vivo efforts; and in vitro experimentation supportive of the in vivo research. The main effort will be on the in vivo approach emphasizing direct optical observation of the events coupled with their quantification. In vivo investigations will include continued study by our existing techniques and instrumentation of the basic mechanisms associated with thrombosis and thrombus susceptibility, vascular fragility, innervation and responsiveness of vascular smooth muscle, platelet-endothelial interactions, and rheological variables. The major new approaches will include (1) development and application of laser doppler heterodyning to measure the velocity profile, shear rate at the vessel wall and along the extent of the thrombus, and the number density ("counts") of platelets and RBC in the measuring volume; (2) study of human and animal blood platelets, human and animal RBC, and human sickle cell disease in one or more animal models developed for this purpose; (3) development and application of microspectrophotometry by transmitted and reflected light and fluorescence to the study of platelets, RBC, and endothelial cells in vitro and in vivo; (4) use of ion-selective microelectrodes to measure intracellular, extracellular, and plasma Na, K and Ca ions activities, as well as pH and PO2, and ascertain the role of these variables in thrombosis and hemostasis; (5) development of techniques to follow the uptake, release, adhesion, aggregation and other biochemical reactions of platelets in vitro and in vivo, determine the biochemical and biophysical mechanisms operative in these reactions, and develop more remote means of detecting the presence of these reactions and means of controlling them.